if it is someone's private bunker, technically breaking and entering.

But as long as he doesn't get caught, and doesn't take or break anything, he's "good". (He'll still be charged with breaking and entering if the owner sees the video and presses charges... he HAS made a video proclaiming himself guilty)

On a related note, if I remember correctly there was a group of urban explorers who helped counter a terrorist attack in Russia at some point.

Can't find anything on the story, but I distinctly remember it being because the Explorers had found an old tunnel that had a secret entrance RIGHT into the place where terrorists had now taken lots of hostages. The Explorers shared the information with the police (knowing they could be charged with all manners of crimes) and the police used the tunnels to erupt in the midst of the terrorists and neutralize them. (if ANYONE can find an article... I can't for the life of me find it, no matter the keyword combination on google XS)